This invention relates generally to land mobile radio systems, and more particularly, to a system providing automatic roaming for land mobile radios.
Land mobile radios (LMRs) may be used to provide communication between different mobile units, for example, between individuals communicating using handheld or vehicle mounted units. Land mobile radio band communication, for example, public safety radio communication (e.g., police, fire department, etc.) is generally available within the VHF, UHF, 700 MHz, 800 MHz and 900 MHz frequency bands. Part of each of these frequency bands is allocated by the Federal Communications Commission (FCC) for public safety communication services and are also referred to as Public Safety Frequency Bands. These communications also may be provided using private land mobile radio services (PLMRS).
In known conventional non-trunking LMR units and corresponding RF communication systems, roaming cannot be provided automatically and is provided manually. In these conventional systems there is no control channel and no guarantee of RF transmissions from a current or adjacent coverage area or base station site. The LMR unit in these conventional systems cannot determine automatically when to switch to another channel, for example, a better channel having stronger signal strength or better coverage.
Additionally, the LMR units in conventional LMR systems are not continuously transmitting and there is no control channel. Therefore, it is not possible in these conventional LMR systems to sample signals to determine, for example, signal strength for use in determining whether to switch to another channel. In these systems the base stations are typically configured as amplifiers and repeaters. Thus, the base stations also cannot provide any continuous control signal for sampling.
In these conventional LMR systems, the switching function is performed manually by a user or operator of the LMR unit. For example, a user may know from experience to change the system or channel when reaching a particular area or location. As another example, the user may hear a degradation in the sound quality of the LMR unit and change the system or channel accordingly. The method based on sound quality is problematic in modem digital conventional systems, for example based on Project 25 standard (TIA 102), wherein digital voice coders (vocoders) are used, because the degradation in sound quality or signal may not be apparent to a user until the signal is almost unusable. Thus, a communication session or link may be dropped or lost with very little warning or indication.
Thus, switching between systems or channels in conventional LMR systems is provided manually. This often results in less than acceptable signal strength or clarity of communication. Further, reliability of communications is affected because the switching is based on the manual action of each individual user. Additionally, manually switching requires additional time and effort by the user, and may increase the likelihood of a possible accident, for example, if a user in a vehicle does not pay attention to the road as a result of having to manually switch the LMR unit.